In modern automatized offices, most of the documents and/or correspondences are transmitted and/or received with facsimile machines. The conventional way that a facsimile machine prints a document can be categorized as follows.
(1) Line-Art Method
This method is basically good for documents containing texts, lines and other symbols only, which are considered bi-leveled images, but it is not able to display the gray levels of images.
(2) Ordered Dither Method
This method can express versatile gray levels. However, texts printed with this method generally contain blurred or coarse edges and moire patterns occurring on the smooth gray area is inevitable during processing documents containing text and images with this method.
(3) Error Diffusion Method
The performance of this method for processing text and image is fallen between the above-mentioned methods. The disadvantages of this method are that noises are present on the white background and the printed image is generally slightly darker than expected. An improved error diffusion method has been proposed to address the background noises and image darkening problems. "IMPROVED ERROR DIFFUSION METHOD" (Chinese Patent Application No. 81108212)
It is very common that the documents to be processed with a facsimile machine may have both text segments and image segments. It is generally up to a user of the facsimile machine to decide with which method a document should be printed. This is, obviously, inconvenient to the user. Further, when a document contains mixed text and image, such as a picture with captions, no matter which method is selected to print the document, quality of a portion of the output will be sacrificed, due to the nature of these prior art methods as described above.
To improve the printing quality, separation of text from image is a remedy for the prior art methods. Theory of text/image separation has been discussed in some academic literatures, such as:
F. M. Wahl, K. Y. Wong and G. R. Casey, "Block Segmentation and Text. Extraction in Mixed Text/Image Documents", Computer Graphics and Image Processing, vol 20, pp. 375-390, 1982.
Also, some patents related to this field have been published, such as U.S. Pat. Nos. 4,411,015, 4,958,238, and 5,073,953.
These methods are generally very complicated in calculation and thus some of them are good only for the recognition of printed text. Further, an optical character recognition system has to be incorporated to perform the separation. U.S. Pat. No. 4,958,238 discloses a method which can be used in facsimile machines. However, since the '238 patent adopts the error diffusion method and uses modified diffusion factors in processing text edges, the processing result thereof has a clear text performance while sacrificing the gray scale of image. This of course is not acceptable for a high quality printing of documents.
It is therefore desirable to provide a simple and effective method to address the afore-mentioned problems and disadvantages.